This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to species signal separation in MRI, such as water and fat signal separation.
Uniform fat separation is necessary for many MRI applications and can be challenging in the presence of main field, Bo, inhomogeneities.
As noted by Reeder et al. in U.S. Pat. No. 6,856,134 for “Magnetic Resonance Imaging With Fat-Water Signal Separation”, in phase and out of phase imaging was first described by Dixon in 1984 and was used to exploit the difference in chemical shifts between water and fat in order to separate water and fat into separate images. Glover further refined this approach in 1991 with a three-point method that accounts for Bo field inhomogeneities. Hardy et al first applied this method to fast spin-echo imaging (FSE) by acquiring three images with the readout centered at the spin-echo for one image and symmetrically before and after the spin-echo in the subsequent two images. These water-fat separation methods have since been combined with both spin-echo (SE) and FSE imaging for many applications (8–13). Several of these three-point approaches acquire one image with the readout centered at the spin-echo and the other two acquired symmetrically on each side of the spin-echo. This approach has the advantage of minimizing the time between refocusing pulses of the fast spin-echo train while providing sufficient time between echoes for phase between water and fat to evolve. Initial descriptions of the relationship between echo spacing and noise performance of water-fat decomposition have been reported, however, these approaches do not fully characterize the theoretical noise performance of water-fat separation methods.
Heretofore, decomposition of water from fat with Dixon methods has not been possible when the proportion of water and fat within a voxel is approximately equal.